Due to the nature of flight, desirable aircraft structures have traditionally had high strength-to-weight ratios (strength efficiency) and stiffness-to-weight ratios (stiffness efficiency). In the last several decades, carbon composite materials were often used in aircraft structures to improve strength and stiffness efficiency in the airframe. Although carbon composite materials do provide weight savings over traditional aluminum structures, carbon composite materials still suffer from a comparatively low compression strength. In a typical unidirectional carbon composite laminate the compression strength is approximately 50% of the tension strength of the material. This is caused by small amplitude waviness in the unidirectional fibers. These small eccentricities in the fibers promote micro buckling of the laminate under compressive loads.
Aircraft wing and blade structures in particular see high bending stresses due to the cantilevered configuration of their structure, and the thin sections required for aerodynamic performance. The bending creates high compression loads in the upper surface of a cantilevered wing or blade structure. Thus, although composite materials do increase the structural efficiency (ratios of strength to weight or stiffness to weight) of aircraft structures over a typical aluminum structure, there is still a large performance gap that can be bridged by increasing the compression strength of the composite laminate.
It is known in the composite materials industry that pultruded composites exhibit significantly higher compression strength than typical fibers pre-impregnated with resin (pre-preg in the industry vernacular) in autoclave cured laminates. The pultrusion process of tensioning fibers and curing them under tension raises the compression strength of the material by over 60%. Pultruded composites also allow lower resin content and therefore a higher fiber volume fraction than a comparable pre-preg structure. Higher fiber volume fractions also lead to higher composite material stiffnesses and strength per unit weight.
FIG. 1 depicts a typical prior art process for making pultrusions. This figure is adapted from “Composite Airframe Structures: Practical Design Information and Data”, by Michael C. Y. Niu, Hong Kong Conmilit Press Ltd., 2005. One or more spools or other sources 102, 104, 106 of tape or other composite material comprising fibers unreel material into one or more wet-out stations or resin tanks 110, 120. The material is then pulled in tension by a pull station 130 before being cured into shape at a heated die station 140 powered by a power source 142. Finished material 150 leaves the die 140 with largely constant cross-section. The finished material 150 comprises pre-cured, pultruded composite fibers. A pre-cured composite material is pultruded when it is cured or formed under tension.
The Niu book, as well as all other extrinsic materials discussed herein, is incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
The value of pultrusions is recognized in the industry; they are employed in composite structures, such as high compression strength areas like wings. FIG. 2 is an illustration of the prior art Genesis 2 sailplane made by Group Genesis™. The aircraft 200 comprises a wing 210 and a fuselage 220. FIG. 3 is a section view of the wing 210. The prior art wing section 300 comprises an airfoil 310 defining the outer boundary. Disposed within the airfoil are a foam vertical stiffener 310 providing bending stiffness, a number of fiberglass laminate sheets 330, 332, 334, 336, and several pultruded rectangular carbon rods 320, 322, 324 that run the length of the wing. Pultrusions have also been used in the wing structures made by large military airframers. However, in prior art known to the inventor, these pultrusions have always been used either as continuous strips running the length of the beam, or as very small rods in an under-stressed structure.
Typical distributed loads on a cantilevered structure such as a wing result in a moment that drops off rapidly from the root to the tip of the structure. A tapered beam structure is often used to take full advantage of possible weight savings where extra structure is not needed. Currently, any tapering of a pultruded structure simply drops off small pultruded sections and uses the material in an under-stressed design, otherwise the abrupt changes in a load bearing members cross-section are transferred to the nearby supporting matrix in too small an area. The resulting stress riser fails the nearby supporting matrix material and ultimately cause a failure of the laminate. Therefore, there is still a need to employ high compression strength composite pultrusions in a highly stressed, tapered laminate.